Processes for copolymerizing ethylene with a vinyl monomer containing a polar group by high-temperature high-pressure free-radical polymerization are well known. Examples of the vinyl monomer containing a polar group include unsaturated carboxylic acids or unsaturated carboxylic acid esters, such as vinyl acetate, (meth)acrylic acid, and (meth)acrylic esters. However, such processes for production by free-radical polymerization require an exceedingly large amount of energy because of the high temperature and the high pressure. In addition, the copolymers produced have low crystallinity due to the branches formed therein in a large number and hence have a drawback in that these copolymers are poor in mechanical and thermal properties (for example, non-patent document 1).
Meanwhile, a polymerization process for polyethylene production using a catalyst such as a metallocene catalyst is known to yield a polyethylene having a linear molecular structure. However, it has been thought that in cases when vinyl monomers containing a polar group are used as comonomers together with a common metallocene catalyst or the like, then these comonomers serve as a catalyst poison to considerably reduce the polymerization activity, making it impossible to obtain a desired copolymer.
It has been reported that copolymerization of ethylene with an acrylic ester, which is a vinyl monomer containing a polar group, using a chromium catalyst yields a linear copolymer having few branches (patent document 1). Consequently, hydrolyzing the ethylene/acrylic ester copolymer gives an ethylene/acrylic acid copolymer which also is linear and has few branches. However, the process for ethylene/acrylic ester copolymer production using a chromium catalyst requires, in a production step therein, aluminum chloride in an amount not less than equivalent to the acrylic ester and necessitates, after completion of the copolymerization, a step for removing substances derived from the aluminum compound. The process hence is industrially inefficient.
It was discovered that in cases when the novel catalyst based on a triarylphosphine or triarylarsine proposed by the present applicants in recent years is used, ethylene and a comonomer containing a polar group, such as an alkyl ester of acrylic acid, are copolymerized to obtain a linear ethylene copolymer without necessitating the use of an aluminum compound in excess (patent document 2). Furthermore, a linear ethylene copolymer obtained by copolymerizing ethylene with a norbornene-based acid anhydride comonomer or the like using the same catalyst and having excellent adhesiveness was proposed by the present applicants (patent document 3).
Moreover, copolymerization of ethylene and acrylic acid using a phosphinesulfonic acid/palladium catalyst was proposed recently as a process in which an aluminum compound is not used in excess likewise (for example, non-patent document 2). Furthermore, a polymer obtained by hydrolyzing a copolymer of ethylene and a t-butyl acrylic acid ester with trimethylsilyl iodide at a temperature of about 40° C. using a similar catalyst to thereby convert the copolymer into a copolymer of ethylene and acrylic acid has been reported (for example, non-patent document 3).